1. Field of the Invention
The present invention relates to a printing control method and a printer, and, in particular, to a printing control method and a printer in which high-quality printing can be achieved even if a sheet feed accuracy for a line feed is relatively low.
2. Description of the Related Art
In a printer such as an impact printer, ideally, a head printing width along a direction perpendicular to a head scanning direction is equal to a pitch of lines printed on a sheet. The term `head printing width` is referred to as the maximum width the head can print. Specifically, in FIG. 1, a printing width `H` of the head along a Y direction is equal to a pitch `L` of lines along the Y direction. In such a case, ordinarily, a mechanism for feeding a sheet one line for a line feed can feed the sheet in a sufficiently high accuracy. Therefore, even if each of heights, along the Y direction, of letters or the like printed on a first line and a second line is equal to the pitch `L` of lines, no gap appears in which nothing is printed between the first and second lines. Thus, high-quality printing can be achieved.
However, the head printing height `H` is not always equal to the line pitch `L`. This is because, ordinarily, a printer has only one head provided therein and therefore the head printing width `H` is fixed for the particular printer. However, in a host control apparatus including a central processing unit (CPU) or the like supplying a printing command and printing data to the printer, a printing format or the like can be freely modified as a result of modifying relevant software. Specifically, by modifying the software in the host control apparatus, independent of the head printing width `H` of the printer, the line pitch `L` and a size of a letter can be freely set and then the printing data is printed according to the set condition. Thereby, printing of a magnified letter having a size, for example, twice, vertically and horizontally, the standard size, printing of various figures, various images, printing of a bar code which extends over several lines, and so forth can be performed. However, when the line pitch `L` is larger than the head printing width `H` a plurality of scanning operations of the head along the scanning direction `X` are required for printing one line.
FIGS. 2A and 2B show a printing control method which may be used for such a case. Hatched boxes shown in FIG. 2A indicate letters, bar codes and so forth printed on a sheet. The head prints a part of a first line with a width `H1` (=`H`) shown in FIG. 2A in a first scanning operation. Then, the sheet is fed the width `H1`, and then the head prints the remaining part of the first line and a part of a second line with a width `H2` (=`H`) shown in FIG. 2A in a second scanning operation. Then the sheet is fed a slight width such that the head may scan the sheet from the top of the second line. Then, the head prints a part of the second line with a width `H3` (=`H`) shown in FIG. 2A in a third scanning operation. At the time, because a part of the second line was already printed in the second scanning operation of the head, in the third scanning operation of the head with the width `H3`, only a part remaining of the second line after the printing in the second scanning operation is printed. As a result of the first, second and third scanning operations with the widths `H1` `H2` and `H3` a width shown in FIG. 2B is scanned. In FIG. 2B, `P1`, `P2`, and `P3` indicate widths which the head prints in the first, second and third scanning operations, respectively.
In one example, the width `H` corresponds to 24 dots, the width which the sheet is fed between the first and second scanning operations is 18/120 inches, and the slight width which the sheet is fed between the second and third scanning operations is 2/120 inches.
The sheet is fed through a sheet feed mechanism. An operation by which the sheet is fed by one line for a line feed and an operation by which the sheet is fed the head width `H` can be performed with relatively high accuracy by the sheet feed mechanism because a width which the sheet is fed is relatively large. However, the above-mentioned slight width which the sheet is fed immediately before the third scanning operation is a very small width in comparison to the width in the case of an ordinary line feed or the like. Feeding the sheet such a very small width through the sheet feed mechanism with a high accuracy is mechanically difficult. Actually, some extent of error is likely to occur.
If such an error occurs and thus, in the example of FIG. 2A, the sheet is fed a width larger than the correct width by an extra width `E` shown in the figure, the head then performs the third scanning operation with the width `H3` from a position, indicated by a broken line shown in FIG. 2A, slightly lower than the top of the second line. In this case, a top part of the second line was already printed in the second scanning operation of the head. However, the printing of the width indicated by `P3` shown in FIG. 2B is performed from a position lower by the error `E`. As a result, a gap `G` in which nothing is printed appears along the head scanning direction `X` within the actually printed letter, bar code and so forth. Such a gap `G` appears in each of cases where the head performs fourth and fifth scanning operations with widths `H4` and `H5` for the same reason. In this example, the error `E` corresponds to 1 or a plurality of dots.
Such a gap `G` appears due to an inaccuracy which occurs because the sheet is fed only slightly. Therefore, this problem does not occur only for the impact printer. In principle, the same problem may occur in various printers such as an ink-jet printer, a thermal printer and so forth.
When such a gap `G` appears, not only is printing quality degraded but also a large problem occurs, in particular, in a case where a bar code is printed. Specifically, if the gap `G` such as that shown in FIG. 2A appears in the printed bar code, when the bar code is read by a bar-code reader and the gap `G` is accidentally scanned horizontally at this time, the bar-code reader cannot determine the presence of the bar code and thus a correct reading of the bar code cannot be performed.
Thus, the bar code cannot be correctly read when a gap or the like is present in the bar code along the horizontal direction, that is, the bar-code reader scanning direction. Therefore, high quality is required in printing the bar code. A printer must be very useful, relatively inexpensive and able to print the bar code without using an especially advanced thus costly printing technology. Therefore, development of a printer is demanded, which printer can solve the above-described problem and can print even such a bar code with a high printing quality.
Further, the above-described gap not only causes a problem in a case of bar code printing but also in cases of printing a magnified letter, various figures, and various images. If such a horizontally extending gap appears in the latter cases, a printing quality may be degraded.